Staggering the tire sizes has been done many times, even on some cars from the factory such as the Corvette. As long as the larger tires are in the rear, you are OK. The main problem I see with doing this is that you can't rotate tires front to rear. So, if you have alignment issues or wear problems on the front, you will chew through tires with no place to put them. The other thing I see with the tires you have is the fronts seem pretty skinny compared to the rear, not that doing that is a problem it just may appear a little funny looking to some people. I personnally like to see some big tires on the rear, but like to put as big of tires on the front as possible (more of a road racer look than a drag racer skinnies look).

235/60/15 on the front on 15X7 Magnums 255/70/15 rear on 15X8 Magnums 1-1/4 coils off the front springs dropped about 2.5"... 2" blocks on the rear custom made with the pins offset to move the axle rearward 1/2" to center the tire in the wheelwell

As Dave stated, different tire sizes (from front to rear) has been done for years in modified vehicles, for the most part.

I believe this is primarily a "take-off" from drag racers who developed the "look" (big and little) as part of their efforts to accelerate quickly with better weight transfer - - getting off the line.

Many people try to use the largest tire and wheel combination possible (without interference) on the rear. As you can imagine, our 71 - 73 mustangs can generally accept a lot more tire than the earlier mustangs.

You should also note that anytime you change the diameter of the rear tires, you also change the accuracy of the speedometer. There are well-known fixes for this but it is something to be considered when making changes.